1. Field of the Invention
This invention relates to a method of reading out a radiation image stored on a stimulable phosphor sheet by exposing the stimulable phosphor sheet to stimulating rays which cause it to emit light in proportion to the stored radiation energy, and photoelectrically detecting the emitted light. This invention particularly relates to a radiation image read-out method wherein preliminary read-out is conducted for approximately detecting the image input information prior to final read-out for reproducing a visible image.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object to have a radiation image stored thereon, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic light-sensitive material or on a display device such as a cathode ray tube (CRT).
The radiation image recording and reproducing system using a stimulable phosphor sheet is advantageous over conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range (latitude) of radiation exposure and further in that the electric signal used for reproducing the visible image can be freely processed to improve the image quality for viewing, particularly for diagnostic purposes. In more detail, since the amount of light emitted upon stimulation after the radiation energy is stored on the stimulable phosphor varies over a very wide range in proportion to the amount of energy stored thereon, it is possible to obtain an image having desirable density regardless of the amount of exposure of the stimulable phosphor to the radiation, by reading out the emitted light with an appropriate read-out gain and converting it to an electric signal to reproduce a visible image on a recording medium on a display device. The electric signal may further be processed as desired to obtain a radiation image suitable for viewing, particularly for diagnostic purposes. This is very advantageous in practical use.
As mentioned above, in the radiation image system using a stimulable phosphor sheet, compensation for deviation of the level of the radiation energy stored on the stimulable phosphor sheet from a desired level can easily be carried out by adjusting the read-out gain to an appropriate value when photoelectrically reading out the light emitted by the stimulable phosphor sheet upon stimulation thereof. Therefore, the quality of the reproduced radiation image is not adversely affected by a fluctuation in radiation dose due to fluctuating tube voltage or MAS value of the radiation source, a variation in the sensitivity of the stimulable phosphor sheet or the photodetector, a change in radiation dose according to the condition of the object, or a fluctuation in the radiation transmittance according to the object, and the like. Also, it is possible to obtain a desirable radiation image even when the radiation dose to the object is low. Further, it is possible to obtain a radiation image having a high image quality of high contrast, high sharpness and low noise, and the like, by converting the light emitted from the stimulable phosphor sheet into an electric signal, and processing the electric signal as desired.
However, in order to eliminate various influences caused by the fluctuation of radiographic exposure conditions and/or to obtain a radiation image having high image quality or a high diagnostic efficiency and accuracy, it is necessary to investigate such image input conditions of the radiation image stored on the stimulable phosphor sheet as, for example, the level of radiation dose used for image recording, or the image input pattern which is determined by the portion of the body (e.g. the chest or the abdomen) or the radiographic method used, such as plain image or contrasted image radiographing, before reproducing the radiation image to a visible image, and then to adjust the read-out gain appropriately or to process the electric signal appropriately based on the detected image input conditions or the image input pattern. The image input conditions and the image input pattern will hereinafter be simply referred to as the image input information when they are referred to generically. It is also necessary to determine the scale factor to optimize the resolution according to the contrast of the image input pattern.
Investigation of the image input information may be conducted prior to the visible image reproduction by use of the method as disclosed in Japanese Unexamined Patent Publication No. 58(1983)-67240. In this method, a read-out operation for detecting the image input information of a radiation image stored on a stimulable phosphor sheet (hereinafter referred to as the preliminary read-out) is conducted in advance by use of stimulating rays having stimulation energy of a level lower than the level of the stimulation energy of stimulating rays used in a read-out operation for obtaining a visible image for viewing, particularly for diagnostic purposes (hereinafter referred to as the final read-out), and thereafter the final read-out is carried out. In the final read-out, the read-out gain is adjusted, and/or an appropriate signal processing is conducted, and/or the scale factor is adjusted to an appropriate value on the basis of the image nput information obtained by the preliminary read-out.
As described above, the level of the stimulating rays used in the preliminary read-out is lower than the level of the stimulating rays used in the final read-out. That is, the effective energy of the stimulating rays which the stimulable phosphor sheet receives per unit area in the preliminary read-out should be lower than the effective energy of the stimulating rays used in the final read-out. In order to make the level of the stimulating rays used in the preliminary read-out lower than the level of the stimulating rays in the final read-out, the output of the stimulating ray source such as a laser beam source may be decreased in the preliminary read-out, or the stimulating rays emitted by the stimulating ray source may be attenuated by an ND filter, an AOM, or the like positioned on the optical path. Alternatively, a stimulating ray source for the preliminary read-out may be positioned independently of the stimulating ray source for the final read-out, and the output of the former may be made lower than the output of the latter. Or, the beam diameter of the stimulating rays may be increased, the scanning speed of the stimulating rays may be increased, or the moving speed of the stimulable phosphor sheet may be increased in the preliminary read-out.
In the aforesaid method, since the image input conditions and the image input pattern of a radiation image stored on the stimulable phosphor sheet can be investigated in advance, it is possible to obtain a radiation image having an improved image quality, particularly a high diagnostic efficiency and accuracy by adjusting the read-out gain and the scale factor on the basis of the detected image input information and by processing the detected electric signal in the manner most suitable for the image input pattern without using a read-out system having a wide dynamic range.
Various methods have been proposed to detect the image input information on the stimulable phosphor sheet based on the preliminary read-out image signals obtained by the preliminary read-out. As one of the methods, it has been proposed to calculate the average value of the preliminary read-out image signals detected within a predetermined region. The average value represents the level of the preliminary read-out image signals as a whole. Therefore, for example, by adjusting the read-out gain in the final read-out based on the average value, it is possible to adjust the level of the final read-out image signal to maintain it always within a correct range.
However, when the image input information on the stimulable phosphor sheet is detected as described above, the problem as described below arises. FIGS. 4A and 4B are explanatory views showing the condition of an object image recorded on the stimulable phosphor sheet. In the case where a direct radiation impingement region B, i.e. the background region where the radiation impinges upon the stimulable phosphor sheet without passing through an object D, as indicated by hatching in FIG. 4A, is recorded within an image region A at which the aforesaid average value is calculated, or the image of a protector C for protecting the object D from the radiation as shown in FIG. 4B or the image of a portion injected with barium contrast media in contrasted image recording of a digestive organ is recorded within the image region A, the image input information on the object D is not obtained accurately. Specifically, the level of radiation energy stored in the stimulable phosphor sheet is very high at the direct radiation impingement region B and very low at the images of the protector C and the portion injected with barium contrast media. Therefore, the average value of the preliminary read-out image signals becomes substatially high or low. However, since such regions are not relaed to the object, the average value obtained in this manner does not correctly represents the image input information on the object.
The same problem arises in all of the cases where the read-out conditions in the final read-out and/or the image processing conditions are adjusted on the basis of a characteristic value of the preliminary read-out image signals which is affected markedly by the direct radiation impingement region or the protector region.